1. Field of the Invention
The invention relates to prostheses for replacement of a portion of a joint such as, for example, the upper portion of the humerus. More particularly, the invention relates to modular components for replacement of a portion of a joint.
2. State of the Art
For the sake of illustration only and without intending to limit the spirit or scope of the invention, the State of the Art and description of the invention that follows will be set forth in the context of a modular prosthesis for replacing a portion of the human shoulder.
Those skilled in the art will readily be able to apply the teachings set forth herein in the exemplary shoulder context, to provide modular components for the replacement of other joints, such as hip joints, etc.,
The human shoulder joint (glenohumeral joint) comprises two major components: the glenoid cavity (glenoid) which is part of the arm and the humeral head (humerus) which is part of the torso. Prosthetic shoulders also comprise glenoid and humeral components. The classical prosthetic humeral component is known as the NEER-type and is a one-piece component which is available in many different sizes for replacement of the upper portion of the organic humerus.
The classical humeral component has a stem which is designed to extend downwardly into a cavity formed within the organic humerus and which is secured with cement or with coatings which promote bone ingrowth to secure the stem. The stem is provided with a generally hemispherical head portion which is configured to replace the head of the organic humerus.
One of the disadvantages of the unitary NEER-type humeral components is the necessity of maintaining a large inventory of different sizes to accommodate different bone sizes in different patients. In addition to the necessity of providing different sized humeral components, it has been recognized that orientation of the humeral head relative to the stem also varies from patient to patient. For these reasons, various modular humeral components have been proposed.
On type of modular humeral component is described in U.S. Pat. No. 5,358,526 to Tornier. The Tornier prosthesis is shown in prior art FIG. 1.
Referring to FIG. 1, the prosthesis 10 generally includes a stem portion 12, a spacer 14, and an hemispherical cap 16. The spacer 14 has opposite tapered studs 14a, 14b.
The stem 12 has a tapered bore 12a which is dimensioned to lockingly engage the stud 14b and the cap 16 has a tapered bore 16a which is dimensioned to lockingly engage the stud 14a. The bore 16a in the cap 16 is also offset from the geometrical center of the hemispherical cap 16 so that the angular orientation of the cap relative to the stud 14a is variable about the axis of the stud 14a.
A plurality of positioning bores 16b are provided in the cap 16, the bores 16b being arranged in a circle about the bore 16a. An index bore 14c is provided in the spacer 14 adjacent to the stud 14a. The angular orientation of the cap 16 relative to the spacer 14 is fixed with the aid of a pin 18 which engages the index bore 14c and one of the positioning bores 16b.
Blind holes 12b and 14d are provided in the stem 12 and spacer 14 respectively and a second pin 20 is provided to engage these blind holes to lock the angular position of the spacer relative to the stem. The stem and spacer are not, however, angularly adjustable.
Though not specifically taught by Tornier, it is presumed that the three components (stem, spacer, and cap) are provided in a variety of sizes and may be mixed and matched to assemble a humeral component which is best suited for a particular patient.
Another approach to modularity in a humeral component is disclosed in U.S. Pat. No. 5,314,479 to Rockwood, Jr. et al. and is illustrated in prior art FIG. 2.
The '479 patent discloses a kit 30 which includes a plurality of different sized stems 32a, 32b, a plurality of different sized "bodies" 34a, 34b, a plurality of different sized collars 36a, 36b, 36c, and a plurality of different sized head members 38a, 38b, 38c. The components are mixed and matched to assemble a humeral component which is best suited for a particular patient.
The kit provided by the '479 patent provides a similar degree of customization as the modular component described by Tornier.
Recent anatomic studies of the humeral head indicate that the head angle, head center, and retroversion may vary significantly among patients.
While the modular components of the prior art are an improvement over the classical NEER-type prosthesis, the adaptability of these modular prostheses is limited, particularly in view of the recent anatomic studies of the humeral head.
Moreover, as may be appreciated from the prior art disclosures, the modular prostheses of the prior art are relatively complex and require elaborate assembly steps.